The kidneys are the chief emunctories of water; and mineral substances which are soluble in water tend for this reason to pass out into the urine. Many of these, as the salts of Iron, Silver, Lead, and Copper, are Astringents, and tend to diminish rather than to increase the amount of urine. But the urine is so necessary and so constant a secretion, that it is very difficult to diminish it in quantity. Saline medicines act as Diuretics under certain conditions. So also do acids and alkalies, and many vegetable substances, which will be presently spoken of. The soluble substances which thus pass off in the urine, and act as Diuretics, are many in number, but they are all uncertain in action, for several reasons. The amount of the urine depends very much on the quantity of water taken into the stomach, or absorbed from the atmosphere through the skin; so that when there is not a sufficient quantity of water in the system, it is impossible for more urine to be formed. This necessary absorption of water depends again upon the pressure of the circulation; when this is too great, it cannot go on, and the urine is diminished. A large increase in any other of the secretions, as that of the bowels,--but particularly of the skin, as in warm dry weather,--hinders diuresis. The secretion of the urine is favoured by those conditions which repress the perspiration; as by coldness of the surface, light clothing, a cold and damp condition of the atmosphere, and the erect posture. In the fourth place, the urine is diminished by causes which impede the circulation, as by congestion of the kidney or liver,--or of the whole venous system, on account of obstruction in the heart,--conditions which produce dropsy. Those medicines act as indirect Diuretics, which, by favouring the removal of one of these hindering causes, tend to allow the secretion of urine to go on as in health. The powerful action of the heart in fevers and inflammations causes such a pressure on the vessels as to retard absorption, and thus hinders diuresis. A hard bounding pulse and hot skin favour sweating more than diuresis, though they often diminish both of these secretions. Both are assisted by a relaxed state of the circulation, which favours the absorption of fluids. Thus venesection, purging, Antimony, or any thing that diminishes a febrile reaction, will help diuresis in such cases. In cases of congestion on account of cardiac disease, Digitalis, which, by powerfully weakening the force of the heart, both diminishes congestion and favours absorption, is a most efficient promoter of the natural function of the kidneys. When the portal circulation is the seat of the obstruction, a mercurial, such as blue pill, which especially acts on the liver, will be a most desirable adjunct to the other remedies employed. Tobacco and Lobelia, like Digitalis, promote diuresis by diminishing the force of the heart.